Nutritional shortages can also give rise to a lagging or incomplete fermentation, naturally depending on the degree of the limitation. As well as causing fermentation to slow down, a shortage of nutritive substances, particularly nitrogen, can give rise to the formation of undesirable end products of metabolism, such as hydrogen sulfide and acetic acid. Under most winemaking conditions, a minimum of 150–250 mg/l assimilable nitrogen is recommended, and some authors have suggested levels as high as 500 mg/l (Jackson, 2000c). Nitrogen utilization usually occurs early during fermentation, before 5% ethanol has accumulated in the medium. Excess nitrogen has been linked to the formation of urea, which has been implicated in the formation of ethyl carbamate, a carcinogen (Bell and Henschke, 2005) (see Section 5.11.5). Mid chain length (C8–C10) saturated fatty acids and phenols are also known to affect yeast if present in high enough concentrations, affecting the progress of fermentations. Most manufactured lyophilized yeast has been grown in the presence of sufficient sulfur dioxide to provide some sort of resistance, but this is strain variable, and the preservative in high enough concentrations will impede membrane function in yeast, but at moderate concentrations (50 mg/l), most fermentations will progress unimpeded. Sulfur dioxide does have some other effects, for example, it inhibits acetic acid production, but favors glycerol synthesis. Elemental sulfur, if present in the medium as a result of antifungal treatment of fruit, can be assimilated, and in the worst case, reduced to hydrogen sulfide, thiols and mercaptans, thus adversely affecting the aroma of the resulting product. It is usually a complex problem. Other factors affecting yeast growth include build up of toxic by products, oxygen depletion, and carbon dioxide and osmotic pressure.